Photochemical Activity Of Photosystem II Research Articles

Photoproduction of hydrogen by sulfur-deprived C. reinhardtii mutants with impaired Photosystem II photochemical activity

Photoproduction of H2 was examined in a series of sulfur-deprived Chlamydomonas reinhardtii D1-R323 mutants with progressively impaired PSII photochemical activity. In the R323H, R323D, and R323E D1 mutants, replacement of arginine affects photosystem II (PSII) function, as demonstrated by progressive decreases in O2-evolving activity and loss of PSII photochemical activity. Significant changes in PSII activity were found when the arginine residue was replaced by negatively charged amino acid residues (R323D and R323E). However, the R323H (positively charged or neutral, depending on the ambient pH) mutant had minimal changes in PSII activity. The R323H, R323D, and R323E mutants and the pseudo-wild-type (pWt) with restored PSII function were used to study the effects of sulfur deprivation on H2-production activity. All of these mutants exhibited significant changes in the normal parameters associated with the H2-photoproduction process, such as a shorter aerobic phase, lower accumulation of starch, a prolonged anaerobic phase observed before the onset of H2-production, a shorter duration of H2-production, lower H2 yields compared to the pWt control, and slightly higher production of dark fermentation products such as acetate and formate. The more compromised the PSII photochemical activity, the more dramatic was the effect of sulfur deprivation on the H2-production process, which depends both on the presence of residual PSII activity and the amount of stored starch.

Promotion of Energy Transfer and Oxygen Evolution in Spinach Photosystem II by Nano-anatase TiO2

Being a proven photocatalyst, nano-anatase is capable of undergoing electron transfer reactions under light. In previous studies we had proven that nano-anatase improved photosynthesis and greatly promoted spinach growth. The mechanisms by which nano-anatase promotes energy transfer and the conversion efficiency of the process are still not clearly understood. In the present paper, we report the results obtained with the photosystem II (PSII) isolated from spinach and treated by nano-anatase TiO2 and studied the effect of nano-anatase TiO2 on energy transfer in PSII by spectroscopy and on oxygen evolution. The results showed that nano-anatase TiO2 treatment at a suitable concentration could significantly change PSII microenvironment and increase absorbance for visible light, improve energy transfer among amino acids within PSII protein complex, and accelerate energy transport from tyrosine residue to chlorophyll a. The photochemical activity of PSII (fluorescence quantum yield) and its oxygen-evolving rate were enhanced by nano-anatase TiO2. This is viewed as evidence that nano-anatase TiO2 can promote energy transfer and oxygen evolution in PSII of spinach.

Effects of Exogenous Spermidine on Photosystem II of Wheat Seedlings Under Water Stress

AbstractThe effects of exogenous spermidine (Spd) on lipid peroxidation, relative plasma membrane permeability, photosystem II (PSII) gene expression and PSII photochemical activity in water‐stressed wheat seedlings were investigated. The decrease in relative water content (RWC), Chl content, and 2,6‐dichlorophenol indophenol (DCIP) photoreduction of PSII, and increases in electrolyte leakage of plasma membranes and malonyldialdehyde (MDA) in water‐stressed leaves was alleviated by Spd pretreatment. Furthermore, Western and Northern blot analysis showed that decreases in the PSII major proteins D1, D2 and LHCII and the transcripts of corresponding genes psbA, psbD and cab were also alleviated by Spd pretreatment under water stress. These results suggest that the application of exogenous Spd protects PSII against water stress at both the transcriptional level and the translational level, and allows PSII to retain a higher activity level during water stress. The protective role of Spd in the photosynthetic apparatus also is discussed.(Managing editor: Ping He)

The dependence of algal H 2 production on Photosystem II and O 2 consumption activities in sulfur-deprived Chlamydomonas reinhardtii cells

Chlamydomonas reinhardtii cultures, deprived of inorganic sulfur, undergo dramatic changes during adaptation to the nutrient stress [Biotechnol. Bioeng. 78 (2002) 731]. When the capacity for Photosystem II (PSII) O 2 evolution decreases below that of respiration, the culture becomes anaerobic [Plant Physiol. 122 (2000) 127]. We demonstrate that (a) the photochemical activity of PSII, monitored by in situ fluorescence, also decreases slowly during the aerobic period; (b) at the exact time of anaerobiosis, the remaining PSII activity is rapidly down regulated; and (c) electron transfer from PSII to PSI abruptly decreases at that point. Shortly thereafter, the PSII photochemical activity is partially restored, and H 2 production starts. Hydrogen production, which lasts for 3–4 days, is catalyzed by an anaerobically induced, reversible hydrogenase. While most of the reductants used directly for H 2 gas photoproduction come from water, the remaining electrons must come from endogenous substrate degradation through the NAD(P)H plastoquinone (PQ) oxido-reductase pathway. We propose that the induced hydrogenase activity provides a sink for electrons in the absence of other alternative pathways, and its operation allows the partial oxidation of intermediate photosynthetic carriers, including the PQ pool, between PSII and PSI. We conclude that the reduced state of this pool, which controls PSII photochemical activity, is one of the main factors regulating H 2 production under sulfur-deprived conditions. Residual O 2 evolved under these conditions is probably consumed mostly by the aerobic oxidation of storage products linked to mitochondrial respiratory processes involving both the cytochrome oxidase and the alternative oxidase. These functions maintain the intracellular anaerobic conditions required to keep the hydrogenase enzyme in the active, induced form.

Photosystem II-based biosensors for the detection of pollutants

Photosystem II (PSII) is the supramolecular pigment–protein complex in the chloroplast, which catalyses the light-induced transfer of electrons from water to plastoquinone (PQ) in a process that evolves oxygen. The PSII complex is also known to bind some groups of (photosynthetic) herbicides, heavy metals and other chemical substances that affect its activity. The objective of this study is to provide an overview of the systems available for the bioassay of pollutants using biosensors that are based on the photochemical activity of PSII. Some applications of the PSII-based biosensors including herbicide, heavy metal monitoring and the detection of radiation in space experiments are reported.

Modulation of D1 protein turnover under cadmium and heat stresses monitored by [ 35S]methionine incorporation

This study examined the response of Photosystem II (PSII) to heavy metal and heat stresses and their combination in two higher plant species, pea ( Pisum sativum) and broad beans ( Vicia faba). The photochemical activity of PSII was monitored as the maximum fluorescence yield of PSII, Fv/Fm, in leaves and as DCPIP photoreduction in thylakoids isolated from control and treated leaves. The pre-treatment of plants with cadmium (1 mM CdCl 2) induced a temporary protection of PSII activity against heat stress (38°C). On the contrary, pre-treatment at the high temperature did not protect PS II against heavy metal (1 mM CdCl 2) stress. The pulse-chase experiments with [ 35S]methionine demonstrated that the two types of stress had different consequences on D1 protein turnover. When the plants were subjected to the cadmium treatment (Cd-treatment), net D1 synthesis was first stimulated, and then inhibited. Heat stress, however, caused an immediate decline of net D1 synthesis. In response to the simultaneous imposition of both stresses, D1 turnover remained stable initially for several hours, but was greatly inhibited under prolonged treatment. Both treatments resulted in a decrease of the D1 level. The symptoms of heavy metal stress were particularly evident when D1 turnover was scarcely stimulated.

Probing Electron Transport through and around Photosystem II in vivo by the Combined Use of Photoacoustic Spectroscopy and Chlorophyll Fluorometry

AbstractPhotoacoustic spectroscopy and chlorophyll fluorometry were used in combination to monitor in vivo several aspects of the photosystem II (PSII) function (O2 evolution, electron transport, and energy storage) in leaves of various plant genotypes. Exposure of barley leaves to 45°C in low light induced a rapid and complete disruption of the O2‐evolving complex of PSII, as shown by the suppression of the photobaric component of the photoacoustic signal. The photoacoustically measured loss of O2 evolution during heat stress (i) was confirmed by polarographic measurements of O2 exchange by the leaf samples, (ii) was shown to be independent of the activity of the carbonic anhydrase, and (iii) was not correlated with a concomitant suppression of the photochemical energy storage and the electron transport activity of PSII. The maintenance of a significant PSII photochemical activity in the absence of O2 evolution is indicative of cyclic electron transport around PSII which bypasses the O2‐evolving complex. The modulation‐frequency dependence of the energy stored during cyclic PSII activity, its inhibition by the herbicide diuron, and its insensitivity to the plastoquinol antagonist DBMIB (2,5‐dibromo‐3‐methyl‐6‐isopropyl‐p‐benzoquinone) indicated that QB, the secondary electron acceptor of PSII, is involved in the electron cycle and that the rate‐limiting step of the cycle is a reaction with a half‐time of ca. 1.5 ms. In the absence of heat stress, overreduction of the plastoquinone pool by strong illumination or by leaf infiltration with DBMIB was not sufficient to induce appreciable cyclic electron transport around PSII. The electron cycle was insensitive to preferential excitation of carotenoids with a blue‐green light and was not significantly affected by the operation of the xanthophyll cycle. Interruption of the electron cycle by diuron during prolonged heat treatment in moderate light was accompanied by a marked peroxidation of the chloroplast thylakoid membranes, as revealed by the appearance of chlorophyll thermoluminescence bands in the temperature range 70–120°C, demonstrating the photoprotective function of electron cycling around PSII.